This invention relates to an infinitely variable hydraulic transmission comprising: a hollow input member and an output member coaxial with the input member; a pump mounted within, and driven by, the input member and including a plurality of pump cylinders and pistons; a multi-cylinder motor mounted within the input member axially offset from the pump and having a stator part secured to the input member for rotation therewith, and a rotor part secured to the output member; and a control device for controlling the supply and return flow of hydraulic liquid from the pump to the motor and vice versa. The transmission is particularly, but not exclusively, suited for driving the internal conveyor screw of a decanter centrifuge, such as the centrifuge described in U.S. Pat. No. 3,971,509.
From French Pat. No. 1,032,245 there is known a transmission of the kind referred to having a variable displacement pump of the radial piston type. The pump comprises a stationary cylinder block with pistons held in engagement with a centrally located eccentric which rotates in synchronism with the input member. The motor is a constant displacement vane type motor having a rotor secured to the output member and a housing formed by a sleeve secured to the input member. The sleeve extends axially into sealing engagement with the outer surface of the stationary cylinder block of the pump, and in the sleeve there are formed two sets of radial ports aligned with the working chambers of the vane motor and the pump cylinders, respectively. Between a casing surrounding the sleeve and rotating together therewith and the outer surface of the sleeve there are defined two longitudinal ducts serving for the supply and return flow, respectively, of hydraulic fluid between the pump and the motor. The delivery rate of the pump, and hence the transmission ratio, may be altered by varying the eccentricity of the eccentric which drives the pump pistons.
In this known transmission large relative velocities occur between the mating cylindrical surfaces of the stationary pump cylinder block and of the sleeve, which latter rotates together with the input member. The concomitant wear of those surfaces in the course of time makes it virtually impossible to maintain a leak-free fit between the surfaces, which however is mandatory for ensuring the reliable function of the transmission.